Lithium batteries, in particular lithium secondary batteries, having such characteristics as a large energy density and a long life span, are used widely as power sources for home appliances such as video cameras and portable electronic devices such as personal notebook computers and cellular phones. Recently, applications into large batteries installed in an electric vehicle (EV), a hybrid electric vehicle (HEV) and the like, are anticipated.
A lithium secondary battery is a secondary battery having a structure in which, during charging, lithium melts out from the positive electrode as an ion and moves toward the negative electrode to be stored and conversely, during discharging, the lithium ion returns from the negative electrode to the positive electrode, and it is known that the high energy density of the battery has its origin in the electric potential of the positive electrode material.
In addition to lithium transition metal oxides such as LiCoO2, LiNiO2 and LiMnO2 having a layer structure, lithium transition metal oxide of the manganese series having a spinel structure (Fd-3m) such as LiMn2O4 and LiNi0.5Mn1.5O4 are known as positive electrode active materials that can be used for lithium secondary batteries (also referred in the present invention to “spinel-type lithium transition metal oxide” or “LMO”).
Owing to low raw material costs and the absence of toxicity, which renders it safe, there is a focus on the spinel-type lithium transition metal oxide (LMO) of the manganese series as a positive electrode active material for a large battery for an electric vehicle (EV), a hybrid electric vehicle (HEV) and the like. In addition, while excellent output characteristics are particularly demanded of a battery for an EV or HEV, on this point, compared to a lithium transition metal oxide such as LiCoO2, which has a layer structure, a spinel-type lithium transition metal oxide (LMO), which allows three-dimensional insertion and desorption of Li ions, has excellent output characteristics.
Meanwhile, when cycles are repeated in a high temperature region (for instance, 45 to 60° C.) with a conventional spinel-type lithium transition metal oxide (LMO), Mn2+ becomes more prone to elution and the eluted Mn2+ deposits on the negative electrode, which becomes a resistance and causes deterioration of the capacity; thus, it has been said that when putting a spinel-type lithium transition metal oxide (LMO) into practical application, the issue lies in the cycle life characteristics in the high temperature region (for instance 45 to 60° C.).
Consequently, in conventional art, various methods are proposed for inhibiting oxygen deficiency to increase the cycle life characteristics in a high temperature region.
For example, in Patent Document 1, a method of inhibiting oxygen deficiency by adding lithium hydroxide after calcination at high temperature and further re-calcining at low temperature is disclosed.
In Patent Document 2, a method of inhibiting oxygen deficiency by calcining the starting materials at the temperature range of 900 to 1000° C. for 5 to 50 hours under oxidative atmosphere and subsequently re-calcining at the temperature range of 600 to 900° C. for 1 to 50 hours under oxidative atmosphere is disclosed.
In Patent Document 3, a method of producing lithium composite oxides by calcining the raw material mixture at high temperature to form a calcined product and re-calcining the calcined product under fluidized condition is disclosed.
Further, under the purpose of providing a lithium secondary battery with excellent high temperature characteristics, especially excellent high temperature storage characteristics, in Patent Document 4, a lithium secondary battery which are provided with a positive electrode active material containing lithium manganate having spinel structure, represented by the general formula (I) LiaMn2-xMxO4-σ (in the formula (I), M represents a substituent element group (Li, Mg, Ca and Ti, or Li and Al) substituting a part of Mn; X represents the substitution amount of each substituent element group (M) in a range of 0<X≦0.5, a represents the amount of Li in a range of 0.1≦a≦1.3, and σ represents the amount of oxygen deficiency in a range of 0≦σ≦0.05, respectively) and specific surface area of 1 m2/g or less is disclosed.